1. Field of the Invention
This invention relates generally to drilling of lateral wells into an hydrocarbon reservoir, and more particularly to the maintaining the wells in a desired position relative to fluid contacts within the reservoir and relative to each other.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, well boreholes are drilled by rotating a drill bit attached at a drill string end. The drill string may be a jointed rotatable pipe or a coiled tube. Boreholes may be drilled vertically, but directional drilling systems are often used for drilling boreholes deviated from vertical and/or horizontal boreholes to increase the hydrocarbon production. Modern directional drilling systems generally employ a drill string having a bottomhole assembly (BHA) and a drill bit at an end thereof that is rotated by a drill motor (mud motor) and/or the drill string. A number of downhole devices placed in close proximity to the drill bit measure certain downhole operating parameters associated with the drill string. Such devices typically include sensors for measuring downhole temperature and pressure, tool azimuth, tool inclination. Also used are measuring devices such as a resistivity-measuring device to determine the presence of hydrocarbons and water. Additional downhole instruments, known as measurement-while-drilling (MWD) or logging-while-drilling (LWD) tools, are frequently attached to the drill string to determine formation geology and formation fluid conditions during the drilling operations.
Boreholes are usually drilled along predetermined paths and proceed through various formations. A drilling operator typically controls the surface-controlled drilling parameters during drilling operations. These parameters include weight on bit, drilling fluid flow through the drill pipe, drill string rotational speed (r.p.m. of the surface motor coupled to the drill pipe) and the density and viscosity of the drilling fluid. The downhole operating conditions continually change and the operator must react to such changes and adjust the surface-controlled parameters to properly control the drilling operations. For drilling a borehole in a virgin region, the operator typically relies on seismic survey plots, which provide a macro picture of the subsurface formations and a pre-planned borehole path. For drilling multiple boreholes in the same formation, the operator may also have information about the previously drilled boreholes in the same formation.
In development of reservoirs, it is common to drill boreholes at a specified distance from fluid contacts within the reservoir. An example of this is shown in FIG. 1 where a porous formation denoted by 5a, 5b has an oil water contact denoted by 13. The porous formation is typically capped by a caprock such as 3 that is impermeable and may further have a non-porous interval denoted by 9 underneath. The oil-water contact is denoted by 13 with oil above the contact and water below the contact: this relative positioning occurs due to the fact the oil has a lower density than water. In reality, there may not be a sharp demarcation defining the oil-water contact; instead, there may be a transition zone with a change from high oil saturation at the top to a high water saturation at the bottom. In other situations, it may be desirable to maintain a desired spacing from a gas-oil. This is depicted by 14 in FIG. 1. It should also be noted that a boundary such as 14 could, in other situations, be a gas-water contact.
In order to maximize the amount of recovered oil from such a borehole, the boreholes are commonly drilled in a substantially horizontal orientation in close proximity to the oil water contact, but still within the oil zone. US Patent RE35386 to Wu et al, having the same assignee as the present application and the contents of which are fully incorporated herein by reference, teaches a method for detecting and sensing boundaries in a formation during directional drilling so that the drilling operation can be adjusted to maintain the drillstring within a selected stratum is presented. The method comprises the initial drilling of an offset well from which resistivity of the formation with depth is determined. This resistivity information is then modeled to provide a modeled log indicative of the response of a resistivity tool within a selected stratum in a substantially horizontal direction. A directional (e.g., horizontal) well is thereafter drilled wherein resistivity is logged in real time and compared to that of the modeled horizontal resistivity to determine the location of the drill string and thereby the borehole in the substantially horizontal stratum. From this, the direction of drilling can be corrected or adjusted so that the borehole is maintained within the desired stratum. The configuration used in the Wu patent is schematically denoted in FIG. 1 by a borehole 15 having a drilling assembly 21 with a drill bit 17 for drilling the borehole. The resistivity sensor is denoted by 19 and typically comprises a transmitter and a plurality of sensors. Measurements may be made with propagation sensors that operate in the 400 kHz and higher frequency.
A limitation of the method and apparatus used by Wu is that resistivity sensors are responsive to oil/water contacts for relatively small distances, typically no more than 5 m; at larger distances, conventional propagation tools are not responsive to the resistivity contrast between water and oil. One solution that can be used in such a case is to use an induction logging that typically operate in frequencies between 10 kHz and 50 kHz. U.S. Pat. No. 6,308,136 to Tabarovsky et al having the same assignee as the present application and the contents of which are fully incorporated herein by reference, teaches a method of interpretation of induction logs in near horizontal boreholes and determining distances to boundaries in proximity to the borehole.
A second situation encountered in reservoir development is illustrated in FIG. 2. Denoted is a borehole 15′ drilled by a drillbit 17′ on a drilling assembly 21′. The reservoir is denoted by 51 and includes a gas-oil contact 57. The objective in drilling here is maintain the borehole at a well below the gas-oil contact. Due to the fact that both gas and oil have relatively high resistivity, it is not possible to ascertain the location of the gas-oil contact using resistivity methods.
U.S. Pat. No. 6,464,021 to Edwards discloses a method for Geosteering using pressure measurements. The method relies upon the fact that vertical fluid pressure gradient (FPG) in a virgin formation depend primarily on the density of the fluid in the formation. Specifically, the vertical FPG in water is approximately 0.5 psi/ft (11.3 kPA/m) for a density of 1.09 g/cc; in oil of density 0.65 g/cc the FPG is 0.37 psi/ft (8.4 kPa/m) while in gas of density 0.18 g/cc the FPG is 0.08 psi/ft (1.81 kPA/m). The method of Edwards includes deploying a number of remote sensing units including pressure sensors into the formation. The deployment is done either from a drill string tool or from an open hole logging tool by drilling into the formation, punching or pressing the remote sensing unit into the formation, or shooting the remote sensing unit into the formation. Using the deployed units, a determination is made of the depth at which drilling of a deviated borehole is to commence. In the absence of hydrodynamic flow, the fluid interface will be substantially horizontal However, there is no discussion in Edwards of a method for maintaining the borehole at the desired depth. All of these are complicated procedures and involve multiple trips down the borehole and/or carrying a number of remote sensing units into the borehole. Another problem not fully addressed in prior art is the spacing of wells for reservoir development.
As a specific example, the desired spacing may be 200 m or so. When surveying is carried out using a gyroscope on a wireline device or a slickline device, a typical accuracy is 1°, which translates into a deviation of 17 m for a 1000 m borehole or 170 m for a 10 km horizontal borehole. With errors of this magnitude, it is difficult to maintain a desired horizontal spacing of 200 m between boreholes. The result is that the reservoir may be oversampledi with boreholes, which costs time and money, or the reservoir may be underampled, resulting in portions of the reservoir being undrained.
It would be desirable to have a method of controlling the drilling of a borehole in a reservoir and maintaining the borehole at a defined distance relative to a fluid interface such as a gas/oil interface or an oil/water interface. Such a method should preferably also be able to maintain the borehole at a specified horizontal spacing relative to a pre-existing borehole. Such a method should reduce the number of interruptions of drilling for the purposes of taking measurements to a minimum. Such a method should also be relatively simple and easy to deploy. The present invention satisfies these needs.